Simultaneous collision-induced transitions in H₂O+CO₂ gas mixtures

A collision induced absorption (CIA) band has been measured near 6000 cm⁻¹ in a spectrum of humidified CO₂ recorded by cavity ring down spectroscopy (CRDS) at low pressure (<1 atm). This absorption adds a contribution to the H₂O+CO₂ continuum mostly originating from far wings of the CO₂ and H₂O resonance lines broadened by collisions with H₂O and with CO₂, respectively. The observed CIA corresponds to a simultaneous excitation of ¹²CO₂ and H₂O colliding molecules in the ν₃ antisymmetric and ν₁ symmetric stretching mode, respectively. CRDS spectra recorded near 5940 cm⁻¹ with a highly enriched ¹³CO₂ sample provide a confirmation of the assignment since the measured CIA isotopic spectral shift (of about −68 cm⁻¹) coincides with that between the ν₃ bands of ¹²CO₂ and ¹³CO₂. The integrated binary coefficient of the two CIA is evaluated and found to be on the order of 2.3 × 10⁻³ cm⁻²amagat⁻². Classical molecular dynamics simulations (CMDS) of the considered CIA are also presented, based on the dominant dipole induction mechanism associated with the vibrational matrix elements of the dipole of CO₂ (ν₃) and isotropic polarizability of H₂O (ν₁). The results of the calculations are found in good agreement with the observations, thus further validating the attribution of the observed CIA structure to the above mentioned double transitions.